Low energy thermochromic image recording device

ABSTRACT

A thermochromic recording device is prepared from reversible photochromic materials. The photochromic material, dissolved in a plastic matrix, is coated onto a substrate. The coating is exposed to light radiation to produce a color therein. The colored layer or film can be rapidly written upon with low energy lasers or light emitting diodes to effect a visible image. Spots having 3 Mu diameters can be recorded with energies as low as 0.043 nanojoules/ Mu 2.

Uited States Patent [1 1 Lorenz et a1.

[451 Sept. 25, 1973 LOW ENERGY THERMOCHROMIC IMAGE RECORDING DEVICE [75]lnventors: Max R. Lorenz, Mahopac;

Rangaswamy Srinivasan, Ossining, both of NY.

[73] Assignee: International Business Machine Corporation, Armonk, NY.

22 Filed: July 28, 1971 21 Appl. No.: 166,879

3,212,898 10/1965 Cerreta 96/90 PC 3.323244 6/1967 Schreiber 3,428,396Megla et a1 350/160 P X Primary Examiner.loseph W. Hartary Att0rneyM.Nanes et a1.

[57] ABSTRACT A thermochromic recording device is prepared fromreversible photochromic materials. The photochromic material, dissolvedin a plastic matrix, is coated onto a substrate. The coating is exposedto light radiation to produce a color therein. The colored layer or filmcan be rapidly written upon with low energy lasers or light emittingdiodes to effect a visible image. Spots having 3 1.1. diameters can berecorded with energies as low as 0.043 nal'lOj0LllS/}L 3 Claims, 3Drawing Figures Patented Sept.- 25, 1973 3,761,942

cownucnow BAND I j hv A A I NORMALN -REVERSE B MODE Y MODE INVENTORS MAXR LORENZ RANGASWAMY SRINIVASAN ATTORNEY LOW ENERGY THERMOCHROMIC IMAGERECORDING DEVICE BACKGROUND OF THE INVENTION 1. Field of the InventionThis invention relates to thermochromic recording devices; morespecifically, the invention relates to the reverse mode operation ofphotochromic recording devices, in which low energy radiation sourcescan be used to inscribe images thereon.

2. Prior Art Much is known about the chemistry and chemical reactions ofphotochromic materials. It is known, for example, that the photocolorreaction of certain of these materials can be reversed thermally.Several of the many studies made on these materials and their reactionscan be found in the following publications and patents:

Photo and Thermochromism 1. Infrared Spectra of l 3 3-trimethyl-6-nitro-8-bromo-spiro [2' l -benzopyran-2, 2'-indoline]Dopedin Polyacrylonitrile Thin Film by M. Lescinsky, Chem. Zvesti, 23(3);194-7 U. S. Pat. No. 3,304,180

Characteristics of the Processer of Coloration and Decolorations ofPhotochromic Substances Derived From Spiropyrans in Polymer, by M. V.Savostjanova et al., Optiko-Mekkar Prom, 33(4), 9-17 1966).

Photographic Process For Repeated Recording and Erasing OpticalInformation, East German Pat. No. 50,988 to Peter Moeckel.

Some Water-Soluble Light Sensitive Spiropyrans,

U. S. Pat. No. 3,320,067 to L. D. Taylor.

Photochromic Compounds Containing Two 2- spiropyran Functions," FrenchPat. No. 1,451,332 to B. Gautron.

"Benzo-B-napthoisospiropyrans And Fluorescent And PhotochromicCompositions Therefrom, U. s. Pat. No. 3,413,234, to J. E. Taylor et al.

Photochromism in Dianthrone And Related Compounds Part III Solutions inAliphatic Hydrocarbons, T. Bercovicci et al., Israel Journal ofChemistry, Vol.7, pages 127-133, (1969).

Photochromic Spiropyrans I. The Effect of Substitutions on the Rate ofRing Closure," E. Berman et al., J. Am. Chem. Soc., 81, p. 5,605 (1959).

Photochromism in Spiropyrans. Part IV, Evidence For the Existence ofSeveral Forms of The Colored Modification, by R. Heligman-Rim et al., J.Phys. Chem. 66, 2465, 2,470 (1965).

Photochromism in Spiropyrans. Part VI [1]Trimethylindolino-benzospiropyran And Its Derivatives," T. Bercovicci etal., Mol. PhotoChem., 1(1), 23-55 (1969).

The above references teach or allude to the use of photochromicmaterials in memory or display devices. However, none of the referenceshave recognized the advantages of operating memory devices using thesematerials in the reverse mode. That is, there is no teaching orsuggestion that low radiation energies could be used to write at veryhigh speeds, i.e., in nanoseconds, by utilizing the reverse chemicalreaction of these materials.

SUMMARY OF THE INVENTION It has been discovered here, that high speedrecording can be effected by exposing the colored state of photochromicmaterials to low energy radiation sources, e.g., lasers or lightemitting diodes, to obtain the colorless state thereof. Consequently,there is provided herein a novel recording device in which there isprovided a substrate having disposed thereon a film containing amaterial which is photochromic and which is also thermochromic. The filmis changed from its colorless state to its colored state by exposing itto light of appropriate wavelength, e.g., ultraviolet light. The coloredfilm is then written upon by exposing it in a predetermined pattern to alow energy radiation source. The recording can be erased simply byre-exposure to light of appropriate wavelength. The invention eliminatesthe need for the use of expensive short wavelength light sources thatare required in prior art photochromic memories.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of the preferred embodiments of the invention as illustratedin the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an energy diagram indicatingthe energy requirements necessary for operating photochromic recordingdevices in their normal mode and in their reverse mode.

FIG. 2 is an isometric drawing depicting the recording device of thisinvention.

FIG. 3 is an isometric drawing depicting another aspect of the recordingdevice shown in FIG. 2

PREFERRED EMBODIMENTS OF THE INVENTION The prior art application ofphotochromic materials to recording devices requires the opticalexcitation of electrons in ground state A (see FIG. 1) to a conductionband state from which they decay to a metastable state B (normal mode).To achieve this it is ncessary to expose the material to a light havinga photon or light energy of -90 Kcal/mole or more. In the presentinvention, advantage is taken of the low energy requirements ofoperating in the reverse mode, i.e., supplying sufiicient heat energy tocause the electrons to go from the metastable state B (colored) to theconduction band back to the ground state A (colorless). It is found thatheat energies as small as 16-30 Kcal/mole can be used by operating thedevice in the reverse mode.

In the practice of the invention, a photochromic material, which in itscolor form or state, i.e., metastable state B, will absorb light in thegreen to far red region of the electromagnetic spectrum, is dissolved ina plastic material. In FIG. 2 a very thin film (I) of this solutionhaving about 30 percent to about 50 percent concentration by weight ofthe polymeric material of the photochromic is coated on a substrate (2).The colorless film (1) (state A) is flooded with ultraviolet radiationfrom a conventional ultraviolet source (3) to its colored form (stateB). The film is now ready to be written upon conventionally with a lowenergy laser (4) having power outputs of as little as l m watt or less.The laser source can be pulsed or continuous radiation can be used.

It should be understood that laser sources having greater power outputscan also be used. The higher the power output the shorter will be therequired time for inscribing information on the colored film. Forexample, energy sources having power outputs of about I m watt requiresabout 300 sees for inscribing a bit of information. On the other hand,an energy source having a power output of about 100 m watt, will require3X10 secs to inscribe the same information.

Photochromics contemplated for use in this invention can be selectedfrom any of a number of classes of materials which demonstrate theproperties of both photochromism and thermochromism. Among the classesof materials which can be used are the spiropyrans and theirderivatives. Typical among these materials is 6 nitro-l 3.3trimethylindolino benzospiropyrane, which has a peak absorption at about5,600A. This compound has the following chemical structure (1) andundergoes the following reaction when exposed to light and heatradiation to produce the resonant structures II: v, 10/16 C II; C Us CH3CH1 CH3 (IIIK Spiropyran derivatives undergoing similar reversiblereactions can be chosen from among 5-OCI-I -,5-OCI-I -6 Cl-, 5-OCH -6Br-I, 6-NO S-OCH -G NO}. 8'-NO and 5,5-Cl -6'-NO derivatives of 1.3.3trimethylindolino benzospiropyran. 1,3 ,3trimethylindolinonapthospiropyran and derivatives. N-methylacridinonaptho-piropyran, dianthrone, dixanthylene,xanthylideneanthrone and their derivrtves can also be used.

The plastic matrix can be prepared from any of a number of polymericmaterials; such as, polymethylmethacrylate polymethacrylate and otheracrylics; polyisobutene, polyvinyl butyral; polyvinylchloridepolyacrytonitrile, polystyrene, polyvinyl alcohol, polyacrylamide, ethylcellulose, polymerized varnish among several other well known filmforming polymeric substances.

A light sensitive film solution is obtained by preparing a dilutepolymer solution in a solvet compatible with the photochromic material.The so prepared solution is saturated with the photochromic material, sothat very thin flms having high concentrations of photochromics can beobtained. A typical film forming solution used in the present inventionis prepared by dissolving 240 mg of polymethylmethacrylate in 6 ml ofacetonitrile, to which was added mg of 1,3,3 trimethylindolinobenzospiropyran. This material has a peak absorption at about 5,6OOA.The solution is then coated onto a substrate by conventional methods,e.g., spinning, dipping, spraying and the like.

The substrate used can be any rigid or flexible material to which thepolymeric film will adhere. It can be transparent as well as opaque. Thefollowing materials can be used as substrates; glass, metals and any ofthe above-mentioned polymeric substances.

In operation, the coated substrate is exposed to a conventional mercuryultraviolet lamp for about 10 minutes to convert the colorless film toits colored state. It is then written upon with a 300 nano sec. pulsedKrypton laser beam having a wave length of 5,680A and an energy of aboutlm watt. A series of 3 1. diameter spots is obtained thereby. The energyrequirement for recording these spots is found to be only about 0.043nanojoules/u. The spots can be erased hy simply re-exposing the film toultraviolet radiation. In prior art photochromic devices where thecolorless film is written upon to record the desired information incolor, a ultraviolet light source is needed, i.e., light having a farshorter wavelength than that used in this invention. Consequently, theenergy required to record 3 p. diameter spots is at least about 5 timesgreater than the requirements of the present invention.

In another aspect of the invention there is provided a recording device(see FIG. 3), in which a metal film (5) is interposed between the activefilm (1) and the substrate (2). The metal film has a relatively flatabsorption response, thus will absorb radiation and by heat conductiontransfer energy to the active film (1). Consequently, the device isdecolorized independent of the wavelength transmitted from the writingradiation source (4). Stated another way, radiation sources used forinscribing information having radiation wavelengths in the infraredregion of the electromagnetic spectra, and which are not necessarilyabsorbed by the photochromic material can be used. For example,injection lasers such as GaAs lasers and light emitting diodes can beused effectively. The metal used can be, for example, Cu, Ni, Ag, Au,and the like. In operation, the active film containing 1.33trimethylindolino benzospiropyran is exposed to ultraviolet radiationfrom radiation source (3) for a time sufficient to change it from itscolorless state to its colored state, e.g.. about 10 minutes. Radiationfrom radiation source 4, e.g., a GaAs injection laser, having awavelength of about 9.000A can be used to inscribe information on theactive film (I). The metal film absorbs the heat energy from radiationsource (4) which is transferred to active film (I). The active film (l)is thus decolorized in the pattern inscribed by radiation from source(4).

It should be understood that the energy required to write can be variedaccording to the absorption spectrum of the particular thermochromicmaterial used. For example, by chemically modifying the particularphotochromic material, the region of radiation absorption can beshifted. Similarly, the addition of light sensitive materials to thematrix material can also change the energy requirements for writing.

What is claimed is:

l. A method for the low energy recording of an image on photochromicmaterials which are thermochromic in their reverse mode which includesthe steps of:

a. preparing a film having a photochromic material dissolved in aplastic matrix, said material being one which becomes colored upon itsexposure to a light source and which, in its colored state, becomesdiscolored upon its exposure to a low energy radiation source, saidmaterial being selected from the group consisting of 1,3,3trimethylindolino benzospiropyran and its 5-OCH 5-OCH -6 Cl, 5-OCH -5Br,6'-NO 5-OCH -6'NO 8.NO and 5,5Cl -6-NO derivatives, 1,3,3trimethylindolino napthospiropyran and its derivatives, and dianthrone,dixanthylene, xanthylideneanthrone and their derivatives;

b. exposing said film to a light energy sufficient to produce a color insaid film; and

c. writing upon said colored film with a low energy laser source whichhas a power output not exdeeding 1 m, watt. 7

2. A low energy image recording device comprising a. a substrate;

b. an active film comprising a photochromic material dissolved in aplastic matrix, said film being disposed on said substrate, saidmaterial being one which becomes colored upon its exposure to a lightsource and which is changed from its colored to its c. a light sourcefor effecting the coloration of said film; and

d. a laser radiation source having a power output not exceeding 1 m wattfor inscribing information on said active film by selectivelydecolorizing said film in its colored state in a predetermined pattern.

3. A low energy image recording device as defined in claim 16 andfurther including:

e. a metal film interposed between said substrate and said active filmfor absorbing radiation and transferrng said absorbed radiation as heatenergy to said active film.

2. A low energy image recording device comprising a. a substrate; b. anactive film comprising a photochromic material dissolved in a plasticmatrix, said film being disposed on said substrate, said material beingone which becomes colored upon its exposure to a light source and whichis changed from its colored to its decolored state upon its exposure inits colored state to a low energy radiation source, said material beingselected from the group consisting of 1,3,3 trimethylindolinobenzospiropyran and its 5-OCH3, 5-OCH3-6''Cl, 5-OCH3-6Br, 6''-NO2,5-OCH3-6''NO2, 8''-NO2 and 5, 5''-Cl2-6''-NO2 derivatives, 1, 3,3trimethylindolino naphthospiropyran and its derivatives, and dianthrone,dixanthylene, xanthylideneanthrone and their derivatives; c. a lightsource for effecting the coloration of said film; and d. a laserradiation source having a power output not exceeding 1 m watt forinscribing information on said active film by selectively decolorizingsaid film in its colored state in a predetermined pattern.
 3. A lowenergy image recording device as defined in claim 16 and furtherincluding: e. a metal film interposed between said substrate and saidactive film for absorbing radiation and transferrng said absorbedradiation as heat energy to said active film.